The present invention relates to a method of combining a received signal of a diversity type radio device and, more particularly, to the method of combining a received signal by delay detecting a received signal of a PSK modulation wave and combining the detected received signal by a maximum ratio combination method(RAKE combination).
The conventional diversity type radio device as described above is used in, for example, a base station of a PHS system, a construction of which is shown in FIG. 3. FIG. 3 is a block diagram of the conventional art. This conventional art is of a space diversity type having two antennas set apart at a predetermined distance. Each 2-branch reception circuit phase-detects a received signal of a PSK (or QPSK) modulation wave by a delay detection method and then weights the 2-branch detection output with a received amplitude level for vector combining with the RAKE combination.
Referring to FIG. 3, a received signal input to antennas 31 and 41 passes through BPFs (Band Pass Filter) 32 and 42, amplifiers at a high frequency with increased width 33 and 43, and BPFs 34 and 44. Each signal is mixed at an output frequency of a local oscillator OSC 50 by mixers 35 and 45 at a first stage, respectively. The mixed signal passes through BPFs 36 and 46, IF amplifiers 37 and 47 through which the signal is mixed down into a primary intermediate frequency. The signal is further mixed at an output frequency of an OSC 51 by mixers 38 and 48 at a second stage, respectively and mixed down into a secondary intermediate frequency, which is input to phase detection sections 39 and 49. The mixers 38 and 48 detect a level amplitude value of the received signal and output amplitude signals 101 and 102.
The phase-detection sections 39 and 49 phase-detect the received signal. The detection output represents an instantaneous phase amount of each symbol as each phase output of the QPSK received signal, i.e., an absolute value of the phase amount.
Each of the detection outputs is input to a combination process section 52 where the amplitude signals 101 and 102 of the respective received signals are operated as weighting factors and vector synthesized, which is output as a combination signal. The combination signal is demodulated into a received data signal in a demodulation section 53.
The conventional method combines the wave detection outputs that have been phase detected. As the detection output represents the absolute value of the phase amount, the respective delay amounts of each group of the element of the reception circuit, BPFs 32 and 42, high frequency amplifiers 33 and 43 and the like are added to the absolute value. The aforementioned delay amount varies dependent on the respective elements. This may cause a phase difference in the detection output between two received signals, resulting in deteriorated combination output characteristics. The level of such deterioration also disperses dependent on the product, leading to inconsistent quality.